We study genes of the rabbit immune system by techniques of molecular biology. We used a restriction fragment length polymorphism (RFLP) of the rabbit T cell receptor beta chain constant region genes (C beta) and an RFLP of rabbit immunoglobulin (Ig) kappa light chains (C kappa) to determine that rabbit C beta and C kappa are not genetically linked as they are in mice, but segregate independently as in man. The C kappa RFLP on the other hand is closely linked to the expression of the K2basl Ig allotype and to a newly discovered RFLP of rabbit V kappa genes. No recombinants have been identified to date. We found evolutionary conservation of both sequence and linkage relationships of gene segments encoding T cell receptor beta chain variable regions. Two of three closely linked rabbit V beta genes have homologs that are closely linked in both mouse and man. A third rabbit V beta gene hybridizes strongly to human DNA. The rabbit probe should identify and allow mapping of a previously unidentified human V beta gene. Like other species, some rabbits have two similar C beta genes but we found that others have three and allotypic forms of both C beta 1 and C beta 2. Stretches of alternating purines and pyrimidines 5' of C beta 1 exon 1, a perfect Chi sequence in exon 1, and Chi with one mismatch in the intron 3' of C beta exon 1, surrounding the allotypic positions, may have contributed to the generation of similar allotypic amino acid sequences in both C beta 1 and C beta 2 of individual rabbits (isoallotypes) by a gene-conversion- like mechanism.